In the production of polyester industrial yarn, often referred to as heavy denier continuous filament yarn, it has often been the practice to produce undrawn filaments of low birefringence such that high draw ratios can subsequently be utilized, thereby obtaining the highest degree of molecular orientation and consequently high tenacities. Such high orientation contributes to the high tenacity of the resulting fibers. It was previously discovered that polyester fibers of low birefringence can be most highly oriented in subsequent drawing steps, but that as filament spinning speeds increase, birefringence tended to increase because of the induced orientation of the filament which occurs in the rapid takeup of the yarn. This results in increased pulling tensions on the yarn in the spinning column, i.e., increases in column draw down. To alleviate the draw down in the column, various procedures have been incorporated into the spinning, including higher extrusion pressures and means for maintaining the filaments molten for a period of time after extrusion. The filaments on quenching and drawing, while achieving high tenacities and modulus and desirable elongations, are found to be deficient in uniformity as measured in percent Uster. This results in a higher standard deviation for the physical properties, such as breaking strength, which is of most critical importance to industrial yarn.
It is therefore an object of the present invention to describe a method for producing a heavy denier industrial polyester yarn of improved uniformity of physical properties.
It is another object of the present invention to provide apparatus for spinning such polyester filament yarns.
It is another object of the present invention to provide a heavy denier industrial polyester filament yarn of high tenacity, improved long and short term percent Uster values and lower standard deviation in breaking strength.
These and other objects will become apparent to those skilled in the art from a description of the invention which follows.